Arylfluorene based universal hosts for solution-processed RGB and white phosphorescent organic light-emitting devices

Abstract

A series of solution-processible arylfluorenes (AFs)/carbazole hybrid compounds (SmeFCz, DMeFCz, SMeOFCz, DMeOFCz, SFFCz and DFFCz) have been designed and synthesized via a simple one-step reaction. These compounds exhibit good thermal stability, excellent solubility and high triplet energy levels, which can be used as universal hosts for solution-processed phosphorescent organic light-emitting devices (PhOLEDs). Employing the developed materials, highly efficient blue (DMeFCz : CE_max: 20.0 cd A⁻¹, EQE_max: 10.0%), green (DMeFCz : CE_max: 26.0 cd A⁻¹, EQE_max: 7.8%) and red (SMeFCz : CE_max: 13.7 cd A⁻¹, EQE_max: 8.2%) PhOLEDs fabricated by spin-coating in different device configurations have been demonstrated. Furthermore, complementary-color, single-emitting layer white phosphorescent device were also obtained. The SMeOFCz-based white device exhibited highly efficient white-light emission with a maximum efficiency of 27.1 cd A⁻¹ (EQE of 12.0%). The state-of-art performance indicates that AFs/carbazole hybrid hosts have a favorable prospect for applications in solution-processed full-color displays and white PhOLEDs.

---

1. Introduction

Organic light-emitting devices (OLEDs) have well-recognized advantages in simple structure, low-driving voltage, flexibility and large area fabrication. They show tremendous commercial applications in flat panel displays and solid state lighting, and have been one of the most attractive projects in optoelectronic information field over the past decades.^1–4 In particular, phosphorescent OLEDs (PhOLEDs) are highly attractive due to their potential for achieving unity internal quantum efficiency through harvesting both singlet and triplet excitons.¹ To achieve high efficiency of PhOLEDs, phosphorescent emitters of heavy-metal complexes are usually dispersed into a suitable host matrix to suppress their intrinsic self-quenching and triplet–triplet annihilation. Generally, highly efficient PhOLEDs could be fabricated by vacuum-evaporation deposition with multilayer device structure, which benefits the fine tuning of charge carrier balance in the emissive layer. To date, vacuum deposited blue, green and red PhOLEDs with external quantum efficiency of over 20% have been reported by several groups.^5–8 However, one of the main challenges facing the industrialization of PhOLEDs is the manufacturing cost because it requires complex technological processes and a large amount of organic materials is wasted. Meanwhile, pixilation using evaporation masks limits large-size scalabilities and high-resolution applications. In contrast, soluble small molecules and polymers can be cast from solution onto large surface areas, thus making low-cost processes such as screen printing, ink jet printing and roll-to-roll coating viable options for production.^9–11 To date, conjugated and nonconjugated polymers have been extensively studied for use as host materials for PhOLEDs.^12–14 Unfortunately, batch-to-batch variations in molecular weight, polydispersity and purity may limit their device efficiencies and practical applications.¹⁵ Additionally, it was difficult to achieve high quantum efficiencies in blue and white PhOLEDs due to the lack of suitable polymer host materials with the high triplet energy levels.¹⁶

As an alternative, small molecule materials with well-defined structures, high levels of purity, and good solubility are expected to be promising solution-processible hosts. Most recently, solution-processible small molecular host materials have attracted much attention due to their great potential for the simple and cost-effective fabrication of highly efficient PhOLEDs.^17–23 For example, Yang developed some silicon-bridged structure molecules as solution-processable hosts for blue PhOLEDs based on bis(3,5-difluoro-2-(2-pyridyl)-phenyl-(2-carboxypyridyl))iridium(III) (FIrpic), giving a maximum luminous efficiency of 23.7 cd A⁻¹ (11.2%).^17–19 Qiu and co-workers synthesized some bipolar host materials by incorporating carbazole and diphenylphosphine oxide units into the star-shaped structure with high triplet energy level (2.82 eV), giving a maximum luminous efficiency of 23.6 cd A⁻¹ (12.2%) for blue PhOLEDs.²⁰ Moreover, with a lower-triplet-energy oxadiazole/triphenylamine hybrid bipolar molecule (2.44 eV) as the host, efficient green (56.8 cd A⁻¹) and red (13.3 cd A⁻¹) devices were also developed.²¹

Despite the great progress that has been made in this field, reports on solution-processable universal RGB hosts, especially those with high device efficiencies, are still limited. This is because the development of such universal hosts is a great challenge. On the one hand, as a host for blue phosphors, its triplet energy level should be at least 2.65 eV to maintain effective energy transfer from host to guest(s).²⁴ On the other hand, for a green or red phosphor, high triplet energy may not be desirable in the host because the enhancement of the triplet energy would inevitably increase the highest occupied molecular orbital (HOMO)/lowest unoccupied energy levels (LUMO) energy gap, leading to large barriers for charge injection. To date, vacuum deposited universal small molecules have been extensively studied for use as host materials for RGB or white PhOLEDs.^25–30 For example, Gong et al. reported the universal bipolar host materials for PhOLEDs which achieved maximum external quantum efficiencies (EQE) of 16.1% for blue, 22.7% for green, 20.5% for orange, and 19.1% for white PhOLEDs.³⁰ Lin et al. reported a novel bipolar host material applied to PhOLEDs of various colors, the device show high EQE (20.7% for red, 20.0% for green, 16.5% for blue, and 15.7% for white) at practical brightnesses.²⁹ Additionally, to generate white light, a single-host structure shows great potentials for the simple and cost-effective fabrication of solution-processed single-emissive layer white PhOLEDs, which facilitates the commercialization of PhOLED lighting and full-color displays. From this point of view, it is necessary to develop solution-processable universal host materials that can be utilized simultaneously for blue, green, and red phosphors.

In our previous works, spirobifluorene^31–35 and arylfluorene derivatives²⁵ have been proved to exhibit good thermal and morphological stability, which is beneficial for amorphous film forming in PhOLEDs fabrication. Some of them have been developed as vacuum-processed universal hosts.^25,32 Arylfluorenes (AFs) have been reported to be synthesized via different methods.^36–40 In this work, a series of (AFs)/carbazole hybrid materials have been one-step synthesized as efficient solution-processable universal hosts with good thermal/morphological stability. By using these hybrid materials as hosts, high luminous efficiencies of 20.0 cd A⁻¹ (10.0%), 26.0 cd A⁻¹ (7.8%), 13.7 cd A⁻¹ (8.2%) and 27.1 cd A⁻¹ (12.0%) have been achieved for blue, green, red and white PhOLEDs, respectively. The easy synthesis, high device efficiencies, high thermal stabilities and good solution processabilities make them promising candidates for optoelectronic materials.

2. Experimental

2.1 Synthesis of materials

SMeFCz and DMeFCz. BF₃·Et₂O (1.1 ml, 5.37 mmol) was added to 100 ml anhydrous CH₂Cl₂ solution of MePhFOH (1.2 g, 4.29 mmol) and 9-octyl-9H-carbazole (1.0 g, 3.57 mmol), and the batch was stirred over a period of 5 h at room temperature. The resulting mixture was washed by water and extracted with CH₂Cl₂. The residue then subjected to silica-gel column chromatography to afford 0.51 g white solid of SMeFCz in ∼26.8% yield, and 1.21 g white solid of DMeFCz in ∼68.8% yield.
SMeFCz. ¹H NMR (400 MHz, CDCl₃) δ 7.93–7.90 (m, 2H), 7.79 (d, J = 7.5 Hz, 2H), 7.48 (d, J = 7.6 Hz, 2H), 7.40–7.22 (m, 8H), 7.18–7.10 (m, 3H), 7.04 (d, J = 8.1 Hz, 2H), 4.22 (t, J = 7.2 Hz, 2H), 2.31 (s, 3H), 1.86–1.78 (m, 2H), 1.37–1.23 (m, 8.8 Hz, 10H), 0.85 (t, J = 6.8 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 152.22, 143.78, 140.76, 140.10, 139.31, 136.34, 136.09, 128.92, 128.15, 127.69, 127.26, 126.31, 125.48, 122.77, 122.51, 120.41, 120.13, 119.65, 118.55, 108.59, 108.36, 65.27, 43.15, 31.81, 29.38, 29.18, 29.03, 27.32, 22.62, 21.00, 14.09. MALDI-TOF m/z: 533.47 [M]⁺; anal. calcd for C₄₀H₃₉N (533.31): C, 90.01; H, 7.36; N, 2.62%; found: C, 90.25; H, 7.43, N, 2.50%.
DMeFCz. ¹H NMR (400 MHz, CDCl₃) δ 7.84 (s, 2H), 7.76 (d, J = 7.6 Hz, 4H), 7.43 (d, J = 7.6 Hz, 4H), 7.34 (t, J = 7.4 Hz, 4H), 7.24 (t, J = 7.4 Hz, 4H), 7.16 (d, J = 5.1 Hz, 4H), 7.12 (d, J = 8.2 Hz, 4H) 7.02 (d, J = 8.0 Hz, 4H), 4.14 (t, 7.2 Hz, 2H), 2.29 (s, 6H), 1.79–1.53 (m, 2H), 1.29–1.21 (m, 10H), 0.84 (t, J = 6.8 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 152.27, 143.82, 140.02, 139.70, 136.10, 136.06, 128.93, 128.05, 127.65, 127.21, 126.30, 126.15, 122.59, 120.08, 119.96, 108.20, 65.28, 43.13, 31.78, 29.33, 29.13, 28.99, 27.27, 22.59, 20.97, 14.06. MALDI-TOF m/z: 787.84 [M]⁺. Anal. calcd for C₆₀H₅₃N (787.42): C, 91.44; H, 6.78; N, 1.78%; found: C, 91.67; H, 6.88; N, 1.61%.

SMeOFCz and DMeOFCz. The synthesis of SMeOFCz and DMeOFCz were similar to that of SmeFCz and DmeFCz, to obtain 0.7 g white solid of SMeOFCz in ∼35.7% yield and 1.0 g white solid of DMeOFCz in ∼56.9% yield.
SMeOFCz. ¹H NMR (400 MHz, CDCl₃) δ 7.92 (d, J = 7.8 Hz, 1H), 7.90 (s, 1H), 7.79 (d, J = 7.5 Hz, 2H), 7.48 (d, J = 7.6 Hz, 2H), 7.43–7.24 (m, 8H), 7.21 (d, J = 8.4 Hz, 2H), 7.14 (t, J = 7.3 Hz, 1H), 6.78 (d, J = 8.8 Hz, 2H), 4.22 (t, J = 7.3 Hz, 2H), 3.77 (s, 3H), 1.86–1.78 (m, 2H), 1.40–1.22 (m, 10H), 0.86 (t, J = 6.8 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 158.27, 152.34, 140.75, 140.02, 139.31, 138.78, 136.42, 129.30, 127.67, 127.25, 126.25, 125.48, 122.75, 122.51, 120.38, 120.13, 119.61, 118.54, 113.52, 108.58, 108.36, 64.90, 55.21, 43.15, 31.79, 29.36, 29.16, 29.02, 27.31, 22.60, 14.07. MALDI-TOF m/z: 549.70 [M]⁺. Anal. calcd for C₄₀H₃₉NO (549.30): C, 87.39; H, 7.15; N, 2.55%; found: C, 87.61; H, 6.98, N, 2.42%.
DMeOFCz. ¹H NMR (400 MHz, CDCl₃) δ 7.83 (s, 2H), 7.77 (d, J = 7.5 Hz, 4H), 7.42 (d, J = 7.6 Hz, 4H), 7.34 (t, J = 7.4 Hz, 4H), 7.24 (t, J = 6.2 Hz, 4H), 7.15 (m, 8H), 6.74 (d, J = 8.6 Hz, 4H), 4.14 (t, J = 7.1 Hz, 2H), 3.75 (s, 6H), 1.79–1.72 (m, 2H), 1.34–1.21 (m, 10H), 0.84 (t, J = 6.8 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 158.24, 152.38, 139.94, 139.69, 138.84, 136.20, 129.20, 127.65, 127.21, 126.25, 126.12, 122.58, 120.10, 119.87, 113.54, 108.22, 64.92, 55.20, 43.13, 31.78, 29.33, 29.13, 28.99, 27.26, 22.59, 14.07. MALDI-TOF m/z: 819.65 [M]⁺. Anal. calcd for C₆₀H₅₃NO₂ (819.41): C, 87.88; H, 6.51; N, 1.71%; found: C, 88.04; H, 6.42, N, 1.75%.

SFFCz and DFFCz. The synthesis of SFFCz and DFFCz were similar to that of SmeFCz and DmeFCz, to obtain 0.90 g white solid of SFFCz in ∼46.9% yield and 0.79 g white solid of DFFCz in ∼46.3% yield. SFFCz: ¹H NMR (400 MHz, CDCl₃) δ 7.92 (d, J = 7.7 Hz, 1H), 7.87 (s, 1H), 7.80 (d, J = 7.5 Hz, 2H), 7.46 (d, J = 7.5 Hz, 2H), 7.43–7.33 (m, 4H), 7.30–7.23 (m, 6H), 7.14 (t, J = 7.4 Hz, 1H), 6.92 (t, J = 8.7 Hz, 2H), 4.23 (t, J = 7.2 Hz, 2H), 1.86–1.79 (m, 2H), 1.38–1.23 (m, 10H), 0.85 (t, J = 6.4 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 151.87, 142.53, 140.76, 140.02, 139.35, 135.99, 129.82, 129.74, 127.77, 127.47, 126.21, 126.12, 125.59, 122.65, 122.55, 120.36, 120.35, 119.53, 118.62, 115.02, 114.81, 64.95, 43.15, 31.79, 31.60, 29.36, 29.16, 29.01, 27.31, 22.60, 14.07. MALDI-TOF m/z: 536.51 [M − H]⁺. Anal. calcd for C₃₉H₃₆FN (537.28): C, 87.11; H, 6.75; N, 2.60%; found: C, 87.41; H, 6.58, N, 2.72%. DFFCz: ¹H NMR (400 MHz, CDCl₃) δ 7.78 (s, 2H), 7.76 (d, J = 4.9 Hz, 4H), 7.41 (d, J = 7.6 Hz, 4H), 7.36 (t, J = 7.4 Hz, 4H), 7.27–7.24 (m, 4H), 7.20–7.16 (m, 8H), 6.89 (t, J = 8.7 Hz, 4H), 4.15 (t, J = 7.3 Hz, 2H), 1.80–1.73 (m, 2H), 1.32–1.21 (m, 10H), 0.84 (t, J = 6.9 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 151.86, 139.95, 139.73, 135.87, 129.71, 129.63, 127.74, 127.44, 126.20, 122.51, 120.21, 115.04, 114.83, 64.95, 31.77, 29.12, 27.31, 27.25, 22.58, 15.00, 14.06. MALDI-TOF m/z: 794.84 [M − H]⁺. Anal. calcd for C₅₈H₄₇F₂N (795.37): C, 87.52; H, 5.95; N, 1.76%; found: C, 87.78; H, 5.78, N, 1.82%.

2.2 General analysis

¹H-NMR and ¹³C-NMR were recorded on a Bruker 400 MHz spectrometer in d-CDCl₃ with tetramethylsilane (TMS) as the interval standard. The MALDI-TOF MS spectra were recorded in reflective mode, with substrates used. Absorption and emission spectra were measured with a Shimadzu UV-3150 and RF-530XPC spectrometer, respectively. Cyclic Voltammetry (CV) were conducted at room temperature on the CHI660E system in a typical three-electrode cell with a platinum sheet working electrode, a platinum wire counter electrode, and a silver/silver nitrate (Ag/Ag⁺) reference electrode. Thermogravimetric analyses (TGA) and differential scanning calorimetry (DSC) were conducted on a Shimadzu DTG-60H thermogravimetric analyzer and a Shimadzu DSC-60A Instrument, respectively, under a heating rate of 10 °C min⁻¹.

2.3 Device fabrication and measurement

The hole-injection material of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS, AI4083) was purchased from H. C. Starck Inc. The electron-transporting material of 1,3,5-tri(m-pyrid-3-yl-phenyl)-benzene (TmPyPB), and the phosphorescent dopants of FIrpic, Tris(2-4(4-toltyl)phenylpyridine)iridium [Ir(mppy)₃], iridium(III)bis(2-phenylquinoline)(acetylaceton) [Ir(pq)₂(acac)] and Tris(2-phenylquinoline-C²,N′)iridium(III) [Ir(2-phq)₃] were purchased from Nichem Fine Technology Co. Ltd. All the above-mentioned materials were used as-received without further purification. In the experiment, the patterned indium tin oxide (ITO) glass substrates were ultrasonically cleaned with detergent, alcohol and acetone, deionized water and then dried at 120 °C in a vacuum oven for more than one hour. After ultraviolet (UV)-ozone treating for 4 min, a 40 nm PEDOT:PSS was spin coated on the ITO substrate and dried at 120 °C in a vacuum oven for 15 min to remove residue solvent. Afterward, the emitting layers (EMLs) were spin-coated on top of PEDOT:PSS from chlorobenzene and then annealed using a hot plate at 100 °C for 20 min to extract residual solvent. The thickness of the EMLs is about 40 nm. Following that, the samples were transferred into a thermal evaporator chamber. The TmPyPB (60 nm), LiF (0.8 nm), and Al (100 nm) were deposited by thermal evaporation under a pressure of 5 × 10⁻⁴ Pa, respectively. The active area of the device is 13.5 mm².

The luminance–current–voltage characteristics of the devices were recorded using a combination of a Keithley source-meter (model 2602A) and a calibrated luminance meter. Electroluminescence (EL) spectra and Commission International de l'Eclairage (CIE) coordinates were obtained using a spectra-scan PR655 spectrophotometer. The thickness of the organic films was measured using a spectroscopic ellipsometry (α-SE, J.A. Wollam Co. Inc.). All the measurements were carried out at room temperature under ambient conditions.

3. Results and discussion

3.1 Synthesis and characterization

All the AFs/carbazole hybrid materials have been synthesized via mild room-temperature one-step Friedel–Crafts reactions. The single-substituted (SMeFCz, SMeOFCz and SFFCz) at 3-position of 9-octyl-9H-carbazole and dual-substituted compounds (DMeFCz, DMeOFCz and DFFCz) at 3,6-position of 9-octyl-9H-carbazole have been obtained, respectively (see the molecular structures in Scheme 1). All of the compounds exhibit good solubility in THF and CH₂Cl₂, which allows for film preparation through solution-process.

Scheme 1 Synthetic route of AFs/carbazole hybrid compounds.

3.2 Thermal properties

The thermal properties of the compounds were determined by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements and the results are shown in Fig. S1 and S2† and summarized in Table 1. All the compounds possess high thermal-decomposition temperatures (T_d) ranging from 366 to 459 °C caused by the large steric effects of aryl-substitution on the sp³-C of the fluorene unit. Dual-substituted compounds show higher T_d than single-substituted ones, possibly due to larger steric effects in dual-substituted compounds. Consistently, the glass transition temperatures (T_g) of dual-substituted compounds is almost two times higher than the single-substituted ones. T_g range from 100 to 134 °C (SMeFCz, SMeOFCz, SFFCz) for single-substituted compounds, while from 224 to 243 °C (DMeFCz, DMeOFCz, DFFCz) for dual-substituted compounds.

Table 1 Physical properties of the AFs/carbazole hybrid compounds^a

	Tg/Td [^oC]	λabs,sol^a [nm]	λabs,film^b [nm]	λem,sol^a [nm]	λem,film^b [nm]	Eg^c [eV]	ET^d [eV]	HOMO/LUMO [eV]
a λabs,sol and λabs,film are absorption maximums in CH3CN solution and film, respectively.b λem,sol and λem,film are emission maximums in CH3CN solution and film, respectively; EHOMO and ELUMO are HOMO and LUMO energy level, respectively, which were determined from cyclic voltammetry.c Eg = ELUMO − EHOMO is the energy band;d ET is the triplet energy; Td is the decomposition temperature; Tg is the glass transition temperature.
SMeFCz	100/383	337, 349	342, 355	361, 378, 401	366, 388, 414, 443	3.86	2.86	−5.59/−1.73
DMeFCz	243/438	339, 351	346, 359	370, 386, 410	372, 388, 414, 441	3.88	2.86	−5.57/−1.69
SMeOFCz	134/401	337, 351	340, 355	362, 378, 400	369, 380, 416, 441	3.78	2.87	−5.56/−1.78
DMeOFCz	228/433	339, 351	347, 360	370, 386, 409	374, 388, 415, 441	3.72	2.86	−5.54/−1.82
SFFCz	125/366	337, 350	340, 357	361, 378, 391	367, 379, 415, 442	3.74	2.87	−5.61/−1.87
DFFCz	224/459	337, 350	340, 357	370, 386, 410	367, 379, 415, 442	3.81	2.86	−5.58/−1.77

---

3.3 Photophysical properties

Due to the non-conjugation linkage between the AFs and carbazole moieties via the sp³C of the fluorene, these AFs/carbazole hybrid materials exhibit similar spectral properties. In CH₃CN solutions (Fig. 1a), maximum absorption of all the materials are similar peaked at 337 nm and 350 nm that attributed to π–π* transition. The emissions peak at 361 nm, 378 nm and 401 nm for single-substituted compounds, and at 370 nm, 386 nm and 410 nm for dual-substituted compounds. Compared to solutions, the film absorption exhibits small red shift of about 3–8 nm with the longer wavelength absorption intensified, and the 0–1, 0–2, 0–3 the film emissions become intensified in film samples (Fig. 1b). The triplet energy levels derived from low-temperature phosphorescence spectra were estimated to be 2.86–2.87 eV (Fig. 2). Their photophysical properties are presented in Table 1.

Fig. 1 Normalized UV-Vis absorption and PL spectra of the AFs/carbazole hybrid compounds in CH₃CN solutions (a) and as thin films (b).

3.4 Electrochemical properties

The electrochemical properties of compounds were investigated by cyclic voltammetry (see Fig. S3†). The HOMO (E_HOMO) and LUMO energy levels (E_LUMO) of the compounds were calculated from cyclic voltammetry and by comparison with ferrocene (4.8 eV). Due to the introduction of carbazole derivatives, quite low E_HOMO values range from −5.54 eV to −5.61 eV have been obtained (see Table 1). The corresponding bandgap E_g are estimated in range of 3.72–3.88 eV.

3.5 Electroluminescent properties of phosphorescent OLEDs

To evaluate the applicability of the AFs/carbazole hybrid compounds as host materials, solution-processed blue phosphorescent devices were firstly fabricated with the architectures of ITO/PEDOT:PSS (40 nm)/15 wt% FIrpic : host (40 nm)/TmPyPb (60 nm)/LiF (0.8 nm)/Al (100 nm). The electron-transporting material TmPyPb, with a high triplet level (E_T = 2.87 eV) and deep HOMO level (−6.7 eV) that can efficiently confine the holes or generated excitons within the emissive region, is selected as the electron-transporting layer.⁴¹ PEDOT:PSS and LiF serve as hole- and electron injecting layer, respectively. As shown in Fig. 3, the normalized EL spectra of the devices exhibit blue emission at around 472 nm that is characteristic of FIrpic emission, and no residual emission from host materials is detected. This demonstrates that complete energy transfer occurs from hosts to FIrpic (Fig. 3). The CIE chromaticity coordinates of the devices are in the range from (0.15, 0.29) to (0.15, 0.31), indicating good color stabilities of these devices. Fig. 4 shows the current density–voltage–luminance characteristics and efficiencies as a function of current density for the blue devices. The detailed device performance is summarized in Table 2. As shown in Fig. 4, the current densities of the SMeOFCz, DMeOFCz and DFFCz-based blue devices are almost identical, the SMeFCz, DMeFCz and SFFCz-based devices exhibit relatively low current densities. The luminous efficiency (CE) and external quantum efficiency (EQE) of these devices are in the range of 13.8–20.0 cd A⁻¹ and 6.9–10.0%, respectively. Among these devices, the device hosted by DMeFCz has the highest device efficiency, with a maximum CE of 20.0 cd A⁻¹ and EQE of 10.0%.

Fig. 2 The phosphorescence spectra of AFs/carbazole hybrid compounds in meTHF solutions at 77 K.

Fig. 3 Normalized EL spectra of blue PhOLEDs based on AFs/carbazole host materials.

Fig. 4 Device characteristics of the solution-processed blue PhOLEDs based on AFs/carbazole host materials: (a) current density–voltage, (b) luminance–voltage, (c) luminous efficiency–current density and (d) EQE–current density.

Fig. 5 Normalized EL spectra of white PhOLEDs at a voltage of 10.0 V based on AFs/carbazole hybrid compounds.

Table 2 Electroluminescence characteristics of single-layer blue, green, red and white PhOLEDs based on AFs/carbazole host materials (V_on is turn-on voltage. L_max is the maximum luminance. LE_max is maximum ​luminous efficiency. EQE_max is maximum ​external quantum efficiency.)

Device	Host	Von [V]	Lmax [cd m^−2]	LEmax [cd A^−1]	EQEmax [%]	CIE [x, y] at 10 V
Blue	SMeFCz	7.5	10 145	18.3	9.1	(0.15, 0.30)
	DMeFCz	8.0	11 281	20.0	10.0	(0.15, 0.29)
	SMeOFCz	6.8	10 736	17.1	8.5	(0.15, 0.29)
	DMeOFCz	6.8	11 416	17.1	8.5	(0.15, 0.31)
	SFFCz	8.1	7815	13.8	6.9	(0.15, 0.31)
	DFFCz	6.8	10 970	14.8	7.4	(0.15, 0.31)
Green	SMeFCz	5.2	12 451	20.9	6.2	(0.29, 0.62)
	DMeFCz	5.1	8179	26.0	7.8	(0.29, 0.62)
	SMeOFCz	5.0	9777	16.5	4.8	(0.31, 0.62)
	DMeOFCz	5.7	7967	21.4	6.3	(0.31, 0.62)
	SFFCz	5.1	11 833	16.1	4.7	(0.29, 0.62)
	DFFCz	5.4	7328	23.8	7.2	(0.29, 0.62)
Red	SMeFCz	6.6	5337	13.7	8.2	(0.61, 0.39)
	DMeFCz	6.0	3915	11.0	6.8	(0.62, 0.39)
	SMeOFCz	6.3	5287	12.9	7.7	(0.61, 0.39)
	DMeOFCz	5.9	4187	10.8	6.8	(0.61, 0.39)
	SFFCz	6.0	4329	10.7	6.4	(0.61, 0.39)
	DFFCz	6.1	4465	10.7	6.7	(0.61, 0.39)
White	SMeFCz	6.9	10 249	22.1	9.4	(0.38, 0.41)
	DMeFCz	6.6	9944	24.1	10.4	(0.39, 0.42)
	SMeOFCz	5.9	10 807	27.1	12.0	(0.36, 0.38)
	DMeOFCz	6.8	9324	24.5	10.8	(0.38, 0.40)
	SFFCz	6.7	11 295	23.0	10.0	(0.37, 0.39)
	DFFCz	6.3	10 206	20.0	8.9	(0.37, 0.38)

---

Based on above results, the solution-processed green- and red PhOLEDs based on the AFs/carbazole hybrid compounds were also evaluated. The device structure was the same as that of blue devices, except that 5 wt% Ir(mppy)₃ and 6 wt% Ir(pq)₂(acac) were used as the green and red dopants, respectively. The performance of the devices is summarized in Table 2. All the green and red phosphorescent devices exhibit stable EL emission within the whole range of driving voltages, and no host emission is observed in the EL spectra due to complete energy transfer. For the green PhOLEDs, the EL spectra of the devices show the similar spectral characteristics with an EL peak at 512 nm, corresponding to the emission of Ir(mppy)₃ (Fig. S4†). The CIE chromaticity coordinates of the green devices are in the range of from (0.29, 0.62) to (0.31, 0.62). Among the green devices, the device hosted by DMeFCz has the highest device efficiency, with a maximum CE of 26.0 cd A⁻¹ and EQE of 7.8% (Fig. S5†). For the red PhOLEDs, all the devices show typical red emission at 600 nm that originates from the guest Ir(pq)₂(acac) (Fig. S6†), with almost no change in the CIE coordinates of (0.61 0.39). The best EL performance was achieved for the device using SMeFCz as host, in which the maximum CE of 13.7 cd A⁻¹ and EQE of 8.2% were obtained (Fig. S7†).

Encouraged by the impressive results obtained from the RGB devices, we fabricated single-EML white PhOLEDs with the structure of ITO/PEDOT:PSS (40 nm)/host : FIrpic : Ir(2-phq)₃ (40 nm)/TmPyPb (60 nm)/LiF (0.8 nm)/Al. In these devices, blue-emissive FIrpic and orange emissive Ir(2-phq)₃ were co-doped host material to form the EML with an optimized doping concentration of 15 wt% for FIrpic and 0.55 wt% for Ir(2-phq)₃. The EL characteristics of white devices are also summarized in Table 2. Fig. 5 shows the normalized EL spectra of the white phosphorescent OLED at the voltage of 10.0 V. The EL spectra exhibit two main peaks of 472 and 579 nm, originating from the emission of FIrpic and Ir(2-phq)₃, respectively. The CIE chromaticity coordinates of these AFs/carbazole hybrid compounds (SMeFCz, DMeFCz, SMeOFCz, DMeOFCz, SFFCz and DFFCz)-based white light devices are respectively calculated as (0.38, 0.41), (0.40, 0.41), (0.36, 0.38), (0.38, 0.40), (0.37, 0.40) and (0.37, 0.38), and they exhibit color rendering index (CRI) of 72, 73, 70, 72, 72, 71 at 1000 cd m⁻², respectively. Although the emission from Ir(2-phq)₃ slightly decreases with increasing driving voltage due to the competition between electron trapping on the yellow-emitting chromophore sites and unperturbed charge through the organic layer, the EL spectra of the white PhOLEDs at various voltages show good white emission (Fig. S8†). Fig. 6 shows the current density–voltage–luminance characteristics and efficiencies as a function of current density for the white devices. The maximum CE and EQE of these devices are in the range of 20.0–27.1 cd A⁻¹ and 8.9–12.0%, respectively. The best EL performance was achieved for the device hosted by SMeOFCz, in which the maximum CE of 27.1 cd A⁻¹ and EQE of 12.0% were obtained.

Fig. 6 Device characteristics of the solution-processed white PhOLEDs based on AFs/carbazole host materials: (a) current density–voltage, (b) luminance–voltage, (c) luminous efficiency–current density and (d) EQE–current density.

4. Conclusion

In summary, a series of AFs/carbazole host materials have been synthesized via one-step reaction for solution-processable PhOLEDs. These new compounds exhibit good thermal properties, high triplet energy, and excellent electrochemical properties. By employing these materials as universal hosts and doped with various color phosphors, high-performance blue, green, red and white PhOLEDs have been successfully realized by solution-processing, showing significant efficiencies up to 20.0 cd A⁻¹, 26.0 cd A⁻¹, 13.7 cd A⁻¹, 27.1 cd A⁻¹, respectively. Therefore, developing and employing the solution-processable efficient small molecule universal hosts, is commonly believed to be an effective way for realizing the high-performance single-color and white PhOLEDs.

Acknowledgements

The project was supported by the 973 projects (2014CB648300, 2012CB723402 and 2012CB933301), the National Natural Science Foundation of China (21373114, 21573111, 61204048, 61136003 and 51173081), the Ministry of Education of China (NCET-13-0872 and IRT1148), the Natural Science Foundation of Jiangsu Province (BK20130037, BK2011760 and BM2012010), the Scientific Research Foundation of NUPT (NY214178 and NY214093), Synergetic Innovation Center for Organic Electronics and Information Displays, and the Project Funded by the PAPD of Jiangsu Higher Education Institutions (YX03001).

Notes and references

1. M. A. Baldo, D. F. O'Brien, Y. You, A. Shoustikov, S. Sibley, M. E. Thompson and S. R. Forrest, Nature, 1998, 395, 151–154.
2. C. D. Muller, A. Falcou, N. Reckefuss, M. Rojahn, V. Wiederhirn, P. Rudati, H. Frohne, O. Nuyken, H. Becker and K. Meerholz, Nature, 2003, 421, 829–833.
3. Q. Wang, J. Q. Ding, D. G. Ma, Y. X. Cheng, L. X. Wang, X. B. Jing and F. S. Wang, Adv. Funct. Mater., 2009, 19, 84–95.
4. H. Uoyama, K. Goushi, K. Shizu, H. Nomura and C. Adachi, Nature, 2012, 492, 234–238.
5. H. Shin, S. Lee, K. H. Kim, C. K. Moon, S. J. Yoo, J. H. Lee and J. J. Kim, Adv. Mater., 2014, 26, 4730–4734.
6. Y. S. Park, S. Lee, K. H. Kim, S. Y. Kim, J. H. Lee and J. J. Kim, Adv. Funct. Mater., 2013, 23, 4914–4920.
7. C.-H. Shih, P. Rajamalli, C.-A. Wu, W.-T. Hsieh and C.-H. Cheng, ACS Appl. Mater. Interfaces, 2015, 7, 10466–10474.
8. C.-H. Shih, P. Rajamalli, C.-A. Wu, M.-J. Chiu, L.-K. Chu and C.-H. Cheng, J. Mater. Chem. C, 2015, 3, 1491–1496.
9. Z. C. Ding, R. B. Xing, Q. A. Fu, D. G. Ma and Y. C. Han, Org. Electron., 2011, 12, 703–709.
10. D. A. Pardo, G. E. Jabbour and N. Peyghambarian, Adv. Mater., 2000, 12, 1249–1252.
11. C. M. Zhong, C. H. Duan, F. Huang, H. B. Wu and Y. Cao, Chem. Mater., 2011, 23, 326–340.
12. X. Gong, W. L. Ma, J. C. Ostrowski, G. C. Bazan, D. Moses and A. J. Heeger, Adv. Mater., 2004, 16, 615–619.
13. Z. Weiguo, M. Yueqi, Y. Min, Y. Wei and C. Yong, Appl. Phys. Lett., 2002, 80, 2045–2047.
14. Z. Chen, C. Y. Jiang, Q. L. Niu, J. B. Peng and Y. Cao, Org. Electron., 2008, 9, 1002–1009.
15. S. Shao, J. Ding, T. Ye, Z. Xie, L. Wang, X. Jing and F. Wang, Adv. Mater., 2011, 23, 3570–3574.
16. X. D. Wang, S. M. Wang, Z. H. Ma, J. Q. Ding, L. X. Wang, X. B. Jing and F. S. Wang, Adv. Funct. Mater., 2014, 24, 3413–3421.
17. S. L. Gong, Q. Fu, Q. Wang, C. L. Yang, C. Zhong, J. G. Qin and D. G. Ma, Adv. Mater., 2011, 23, 4956–4959.
18. S. L. Gong, C. Zhong, Q. Fu, D. G. Ma, J. G. Qin and C. L. Yang, J. Phys. Chem. C, 2013, 117, 549–555.
19. S. L. Gong, Q. Fu, W. X. Zeng, C. Zhong, C. L. Yang, D. G. Ma and J. G. Qin, Chem. Mater., 2012, 24, 3120–3127.
20. W. Jiang, L. Duan, J. Qiao, G. F. Dong, L. D. Wang and Y. Qiu, Org. Lett., 2011, 13, 3146–3149.
21. M. R. Zhu, T. L. Ye, X. He, X. S. Cao, C. Zhong, D. G. Ma, J. G. Qin and C. L. Yang, J. Mater. Chem., 2011, 21, 9326–9331.
22. X. W. Zhang, X. Guo, Y. H. Chen, J. Y. Wang, Z. F. Lei, W. Y. Lai, Q. L. Fan and W. Huang, J. Lumin., 2015, 161, 300–305.
23. H. Liu, Q. Bai, L. Yao, D. Hu, X. Tang, F. Shen, H. Zhang, Y. Gao, P. Lu, B. Yang and Y. Ma, Adv. Funct. Mater., 2014, 24, 5881–5888.
24. J. Lee, N. Chopra, S. H. Eom, Y. Zheng, J. G. Xue, F. So and J. M. Shi, Appl. Phys. Lett., 2008, 93, 123306.
25. X.-H. Zhao, Z.-S. Zhang, Y. Qian, M.-D. Yi, L.-H. Xie, C.-P. Hu, G.-H. Xie, H. Xu, C.-M. Han, Y. Zhao and W. Huang, J. Mater. Chem. C, 2013, 1, 3482–3490.
26. E. Mondal, W.-Y. Hung, H.-C. Dai and K.-T. Wong, Adv. Funct. Mater., 2013, 23, 3096–3105.
27. H.-H. Chou and C.-H. Cheng, Adv. Mater., 2010, 22, 2468–2471.
28. W.-Y. Hung, L.-C. Chi, W.-J. Chen, E. Mondal, S.-H. Chou, K.-T. Wong and Y. Chi, J. Mater. Chem., 2011, 21, 19249–19256.
29. M.-S. Lin, L.-C. Chi, H.-W. Chang, Y.-H. Huang, K.-C. Tien, C.-C. Chen, C.-H. Chang, C.-C. Wu, A. Chaskar, S.-H. Chou, H.-C. Ting, K.-T. Wong, Y.-H. Liu and Y. Chi, J. Mater. Chem., 2012, 22, 870–876.
30. S. Gong, Y. Chen, J. Luo, C. Yang, C. Zhong, J. Qin and D. Ma, Adv. Funct. Mater., 2011, 21, 1168–1178.
31. Y. Qian, G. H. Xie, S. F. Chen, Z. D. Liu, Y. R. Ni, X. H. Zhou, L. H. Xie, J. Liang, Y. Z. Zhao, M. H. Yi, Y. Zhao, W. Wei and W. Huang, Org. Electron., 2012, 13, 2741–2746.
32. M.-L. Sun, S.-Z. Yue, J.-R. Lin, C.-J. Ou, Y. Qian, Y. Zhang, Y. Li, Q. Wei, Y. Zhao, L.-H. Xie and W. Huang, Synth. Met., 2014, 195, 321–327.
33. J. Zhao, G. H. Xie, C. R. Yin, L. H. Xie, C. M. Han, R. F. Chen, H. Xu, M. D. Yi, Z. P. Deng, S. F. Chen, Y. Zhao, S. Y. Liu and W. Huang, Chem. Mater., 2011, 23, 5331–5339.
34. L. H. Xie, R. Zhu, Y. Qian, R. R. Liu, S. F. Chen, J. Lin and W. Huang, J. Phys. Chem. Lett., 2010, 1, 272–276.
35. Y. Qian, Y. Ni, S. Yue, W. Li, S. Chen, Z. Zhang, L. Xie, M. Sun, Y. Zhao and W. Huang, RSC Adv., 2015, 5, 29828–29836.
36. P.-I. Shih, C.-L. Chiang, A. K. Dixit, C.-K. Chen, M.-C. Yuan, R.-Y. Lee, C.-T. Chen, E. W.-G. Diau and C.-F. Shu, Org. Lett., 2006, 8, 2799–2802.
37. K. T. Wong, Y. M. Chen, Y. T. Lin, H. C. Su and C. C. Wu, Org. Lett., 2005, 7, 5361–5364.
38. A. Tomkeviciene, J. V. Grazulevicius, D. Volyniuk, V. Jankauskas and G. Sini, Phys. Chem. Chem. Phys., 2014, 16, 13932–13942.
39. J. Keruckas, D. Volyniuk, T. Matulaitis, J. V. Grazulevicius and V. Jankauskas, Synth. Met., 2015, 199, 365–371.
40. A. Kruzinauskiene, A. Matoliukstyte, A. Michaleviciute, J. V. Grazulevicius, J. Musnickas, V. Gaidelis and V. Jankauskas, Synth. Met., 2007, 157, 401–406.
41. S. J. Su, T. Chiba, T. Takeda and J. Kido, Adv. Mater., 2008, 20, 2125–2130.

---

Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra19231e

‡ These authors contributed equally to this work.

---